Fix posts number

[majoskorec]

Case
discussion A have 3 posts with numbers : 1,2,3
discussion B have 2 posts with numbers : 1,2
with time order A1,A2,A3,B1,B2

then you "Delete Forever" post 2 at discussion A

discussion A have 2 posts with numbers : 1,3
discussion B have 2 posts with numbers : 1,2
with time order A1,A3,B1,B2

when you merge discussion B to A it "renumber"
B2 => A4
B1 => A3
A3 => A2
A1 => A1

but second sql update (B1 => A3) throws error that unique index A3 already exist

Changes proposed in this pull request:
we will fix posts number on target discussion before merging

[imorland]

Thanks for this PR! This has been an issue for some time, with various attempts at resolving it.

I'll get this merged, I'm sure it will help for some scenarios!